1. Field of the Invention
The present invention relates generally to a communication method in a CDMA (Code Division Multiple Access) communication system, and in particular, to a method of implementing a dedicated control channel (DCCH) handoff.
2. Description of the Related Art
One method of increasing system capacity in a CDMA communication system, is through channelization, wherein orthogonal codes are used. The orthogonal codes can be Walsh codes. The orthogonal channelization is applied to a forward link in the IS-95 standard, for example.
To provide orthogonal channelization to the forward link in an IS-95 communication system, common channels (a pilot channel, a paging channel, and a sync channel) are spread using predetermined orthogonal codes and a traffic channel assigned for a user is spread using an idle orthogonal code. Then, a serving base station (BS) notifies a mobile station (MS) of the index of the assigned orthogonal code and the MS despreads a received signal using the same orthogonal code.
Each channel on the IS-95 forward link is convolutionally encoded and a modulator performs BPSK (Bi-Phase Shift Keying) modulation. The bandwidth used is 1.2288 MHz and the data rate is 9.6 kbps in the IS-95 communication system. Therefore, 64 channels (=1.2288M/(9.6 kxc3x972)) on an IS-95/IS-95A forward link are distinguished by 64 orthogonal codes. The number of available orthogonal codes is obtained after a modulation scheme and a minimum data rate are determined.
In the IS-95 system, a BS uses a soft handoff when directing a MS from one forward traffic channel to another forward traffic channel having the same frequency assignment. The soft handoff occurs when an MS moves out of the coverage area of a serving BS in an old cell and enters a neighboring BS in a new cell. During soft handoff, the system establishes a link between the MS and the new BS without disconnecting the MS from the serving BS and switches the call to the new BS.
The IS-95 system has a traffic channel as the only dedicated channel. However, the next-generation of CDMA communication systems will increase channel transmission capacity by increasing the number of channels available to users. To do so, the next-generation of CDMA links are comprised of dedicated channels including traffic channels and a dedicated control channel, and common channels including a pilot channel, a common control channel, and a paging channel. The traffic channels include a fundamental channel (FCH) used for voice transmission and a supplemental channel used for transmission of packet data.
Handoff is defined for a fundamental channel only in the conventional CDMA communication system and therefore a handoff cannot be implemented for a dedicated control channel in the next-generation of CDMA communication systems. Therefore, a new handoff procedure must be suggested to the standardization organization, that implements the new channels defined in the next-generation of CDMA communication systems. See Korea Patent No. 98-4498 for a detailed description of the dedicated control channel and Korea Patent No. 98-11381 for a detailed description of forward and reverse channels used in the CDMA 2000 system.
It is, therefore, an object of the present invention to provide a channel assigning device and method in a CDMA communication system, wherein a dedicated control channel is additionally assigned to switch a call to a new BS during handoff.
It is another object of the present invention to provide a channel assigning device and method in a CDMA communication system, in which a channel is additionally assigned using a quasi-orthogonal code due to lack of available orthogonal codes to switch a call to a new BS during handoff.
It is a further object of the present invention to provide a channel assigning method in a CDMA communication system, wherein an MS analyzes a handoff direction message (HDM) received on a traffic channel and assigns a reception channel corresponding to the transmission channel according to a channel ID specified in the handoff direction message.
It is still another object of the present invention to provide a method of forming a handoff direction message to be transmitted to an MS by a BS during a handoff in a CDMA communication system.
To achieve the above objects, there is provided a handoff method in a CDMA communication system using Walsh codes and quasi-orthogonal codes. To implement a handoff, a new base station to which a call is switched notifies a serving base station of an available quasi-orthogonal code, if the new base station has no available Walsh codes. The serving base station transmits a handoff direction message to a mobile station and the mobile station communicates with the new base station using the quasi-orthogonal code included in the handoff direction message.